Plasma screening in mid-charged ions observed by K-shell line emission

TL;DR

This work probes plasma screening in mid-charged copper ions under warm dense matter conditions by tracking K-shell line shifts (Kα, Kβ, Kγ) and hollow-ion transitions using resonant pumping with a narrow-band XFEL. A detailed FAC-based atomic model (isolated and with Stewart-Pyatt screening) is confronted with experiment, enabling direct extraction of screening as a function of effective charge state and shell occupancy. The key finding is that the Stewart-Pyatt model underestimates screening across the studied charge-state range (up to $K_ ext{eff} ightarrow 26$) unless implausibly low temperatures are invoked, underscoring the need for improved IPD/CL treatments in dense plasmas. The results provide a rich dataset (including hollow-ion lines) to guide development of next-generation screening codes and advance understanding of atomic physics in Warm Dense Matter.

Abstract

Dense plasma environment affects the electronic structure of ions via variations of the microscopic electrical fields, also known as plasma screening. This effect can be either estimated by simplified analytical models, or by computationally expensive and to date unverified numerical calculations. We have experimentally quantified plasma screening from the energy shifts of the bound-bound transitions in matter driven by the x-ray free electron laser (XFEL). This was enabled by identification of detailed electronic configurations of the observed Kα, K\b{eta} and Kγ lines. This work paves the way for improving plasma screening models including connected effects like ionization potential depression and continuum lowering, which will advance the understanding of atomic physics in Warm Dense Matter regime.

Figures (14)

• Figure 1: Schematic depiction of observed transition chains: emission above K edge (a), K$\upalpha$ emission driven by K$\upbeta$ absorption (b), and K$\upalpha$ and K$\upalpha_\mathrm{h}$ driven by K$\upbeta_\mathrm{h}$ (c). K$\upalpha$ transitions calculated by the FAC code (d). Each circle is a single transition with size corresponding to its oscillator strength; the transitions are grouped according to charge state (y-axis) and L-shell occupancy (color), and a weighted mean for each group is shown by a vertical marker.
• Figure 2: Experimental spectra for beam energy density 110 kJ/cm$^2$ (a) with identified resonances (stars with color corresponding to L-shell occupancy), edges (white bars), and elastic scattering (white circles). The fitted intensity and position of K$\upalpha$ L6 is shown in (b) and (c).
• Figure 3: Map of spectral lines in Cu. Black outlined stars show emission lines and bars absorption energies observed in the experiment. Translucent symbols are calculated by the FAC code for isolated atom. Color of symbols corresponds to L shell occupancy.
• Figure 4: Observed line shifts (a...e) and edge positions (f) for energy density 110 kJ/cm$^2$. Stars are measured by emission, triangles by absorption with errorbar corresponding to the XFEL bandwidth. The color corresponds to L shell occupancy with same coding as Fig. 3. Grey lines are predictions by SP model in FAC for various temperature assumptions, labeled in (d) for all panes. The bands in (f) show the calulated edges for various M-shell occuapncy, width of band contains data for temperaterus between 5 and 107 eV.
• Figure 5: Temperatures of the plasma calculated by the CR codes SCFLY and Cretin (a). Solid lines are temperatures during the peak of the XFEL beam, dotted lines are the maximal temperatures, reached toward the end of the pulse. Stars indicate temperatures estimated from the XRTS data. Observed (b) and predicted (c) Stark shifts as a function of energy density and plasma temperature, respectively. Blue band in (b) is a linear fit to the data, and is transferred into (c) by using the measured temperature-energy density relation in (a).